The indecent assault occurred between 9.30am and 10am at Falcon Bay Beach on Spinaway Parade.
A 70-year-old woman was indecently assaulted by a man inside the toilet block.
The man is described as being in his late 20s to early 30s, with fair skin, slim build and shoulder length brown hair.
He was wearing a tangerine/red coloured T-shirt and a khaki floppy brimmed hat.
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Detectives would like to speak to anyone who was in the Falcon Bay Beach area, who may have seen the man, particularly any men around the public toilet area.
Call Crime Stoppers on 1800 333 000, or report the information online at www.crimestopperswa.com. au.